Problem: Solve for $a$ : $19 = 21 + a$
Subtract $21$ from both sides: $ \begin{eqnarray} \\ \hphantom{19 {- 21}} &\hphantom{=}& \hphantom{a} \\ 19 &=& 21 + a \\ \\ {-21} && {-21} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 19 {- 21} &=& a \\ \end{eqnarray} $ Simplify. $ a = -2$